All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
There is no doubt that bad moral conduct is a reason for making people keep away from a person. What you mentioned about your father that he supplicates against people and their children, and supplicates against his grandson to die, and prevents you from being kind and dutiful to your mother, and so forth, then all this is an injustice and a bad moral conduct, and this is not permissible. He should repent to Allaah. What he mentioned that he has the right to oblige you to marry another woman is not correct, and he has no right to ask you to divorce your wife. If he orders you to divorce her –without a sound reason –you are not Islamicly obliged to divorce her as we clarified in Fatwa 84808.
Besides, following people’s faults is not among the moral conduct of the believers. Furthermore, what he mentioned that you are obliged when your father dies to take care of your step-mother is also not correct and you are Islamicly not obliged to do so. As regards taking care of your brothers and sisters, then there are long details about this according to the kind of care: is it spending on them or running their affairs? Also, the situation differs between your full brothers [and full sisters] and brothers and sisters on the father’s side only. You might be obliged to run the affairs of the minors amongst them, but you are not obliged to spend on them. Anyway, you should not be preoccupied by this before it happens.
Therefore, we advise you to continue being kind and dutiful to your father and advise him in a soft and gentle manner. You should know that his ill-conduct is not a sound reason for you to severe ties with him. Also, your sister should be kind and dutiful to her father and you are both obliged to obey him if he does not order you to disobey Allaah.
For more benefit on the rights of parents, please refer to Fataawa 90751 and 87019, and to Fatwa 95504 on how to reconcile between the parents when the father is determined to divorce.
Allaah Knows best.